Rapid Infiltration Characteristics in Cracked Clay Soils in Texas Playas.

Randall clay (very-fine, smectitic, thermic Ustic Epiaquerts) is a shrink-swell soil typical of Texas Panhandle playa floors. Our objective was to document the time-related sealing properties of dry, cracked Randall clay when subjected to a rapid infiltration event. Twenty-six soil cores of 76-mm diameter by 152-mm length were collected from two playa lakes in Randall County, Texas, and the clay mineralogy determined. The cores were air dried for one month, and cracking was documented by depth using computer-aided tomography (CAT) scans at 12.7-mm increments before and after infiltration events. The soil cores were subjected to infiltration and saturated hydraulic conductivity tests in a flexible wall permeameter. Saturated hydraulic conductivities varied from 5.4x10^-8 to 1.2x10^-10 m s^-1 with geometric means of 6.8x10^-10 and 2.3x10^-9 m s^-1 at the two playas. After the cracks sealed due to soil expansion upon wetting, very low infiltration rates were observed. Cumulative infiltration after 60 minutes varied from 1.5 to 16.0 mm, and the cores effectively sealed before water traveled more than 81.3 mm. These infiltration rates are six to eight orders of magnitude, and the cumulative infiltration one to two orders of magnitude, less than values reported in earlier research. The pre-infiltration CAT scans revealed considerable variability in the amount, vertical and horizontal distribution, and continuity of the cracking patterns. After infiltration, all cracks were sealed. These results suggest that in some playas there is less recharge potential through playa floors than previously estimated.